Other than one-piece practice or driving range balls, golf balls are generally available as two-piece and three-piece balls.
Three-piece or wound balls, which are preferred by professionals and low handicap amateurs, comprise either a solid rubber or liquid filled rubber center or core, covered by elastic windings. Wound balls typically have a cover of balata, which exhibits enhanced playability properties, but lacks the durability required for repetitive play, or an ionomer resin. Wound balls provide better golfers with a higher spin rate and more control than two-piece balls. In contrast, two-piece golf balls, used by the average amateur golfer, provide a combination of durability and maximum distance. Two-piece balls have a core formed of a single solid sphere, which is typically comprised of a polybutadiene base compound, and an ionomer resin cover that encloses the core.
Ionomer resins are polymeric materials containing interchain ionic bonding. They are sold by E. I. Du Pont de Nemours & Company under the trademark SURLYN.RTM., and by Exxon Corporation under the trademark ESCOR.RTM. and the trade name IOTEK. Because of their toughness, durability, and flight characteristics, ionomers have become the material of choice for golf ball covers. Golf ball covers made from ionomers possess the durability for repetitive play that is lacking in balata covers.
Ionomer resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic or methacrylic acid. A portion of the pendant acid groups in the copolymer are neutralized with a metal, such as sodium or zinc to produce a thermoplastic elastomer with enhanced durability compared to balata.
At present there are more than 50 commercial grades of ionomers available from Du Pont and Exxon. The properties of these ionomers vary over a wide range according to the type and amount of metal cations present in the polymer, the molecular weight, the composition of the base resin, and additives, if any, that are included in the resin. Research continues in an effort to develop a golf ball cover composition having the improved distance of an ionomer cover with the playability of balata.
The distance a golf ball will travel is determined by a number of factors, including the coefficient of restitution (COR), ball size, weight, club head speed, angle of trajectory, and the dimple pattern, which controls the aerodynamics of the ball. The COR and the dimple pattern are factors that are under the control of the manufacturer, and, therefore, are of particular concern to the manufacturer.
The COR of a golf ball can be measured by propelling a ball at a hard, unyielding surface at a known speed, and measuring its speed immediately after impact. The COR is the ratio of the speed after impact to the speed before impact, and can vary from zero for a totally inelastic collision to one for a totally elastic collision. For macroscopic objects, the COR is always less than one.
The COR for a one-piece ball is a function of the composition only. However, in two-piece and three-piece balls, the COR is a function of the core and the cover.
Although the United States Golf Association (USGA) does not place any specific limitations on the COR, a USGA approved ball cannot exceed an initial velocity of 255 feet/second under specified test conditions, which places an effective limitation on the COR. Therefore, golf ball manufacturers seek to produce golf balls with a high COR so that the ball will approach or equal 255 feet/second in the USGA test without violating the speed limitation.
In their attempts to produce golf balls that provide the maximum distance while remaining within the USGA rules, golf ball manufacturers have produced various blends of ionomer resins. However, a need still exists for an ionomer blend for golf ball covers that provides the durability required for repetitive play, the COR required to provide maximum distance under the USGA rules, and the playability and control desired by better golfers. The present invention provides one such cover composition.